For a warship or other tactical engagement system, any intrusion of a guarded tactical space by non-friendly aircraft is a serious threat that must be dealt with accordingly such as by using various AAW (Anti-Aircraft Warfare) engagement means.
Conversely, preventing engagements with neutral aircraft, and in particular with commercial airliners, is a significant concern, particularly in military operations. Verbal alerts provided to the aircrew have been traditionally used to alert the aircrew to avoid intrusion into a guarded tactical space and various alert mechanisms have been used to supplement the verbal warning method. The verbal alerts may be made using radio or other known signals. Current advances that utilize Secondary Surveillance Radar (SSR) and heightened awareness are methods for mitigating, but not completely eliminating, the risk of engagement with neutral aircraft. It would be desirable to employ orthogonal methods for preventing the engagement of neutral aircraft because the goal of avoiding firing upon or otherwise engaging neutral aircraft, is more likely to be achieved when multiple, orthogonal methods are used to warn a neutral aircraft to avoid entering a protected airspace.
The Traffic Alert and Collision Avoidance System (TCAS) is an aircraft collision avoidance system based on SSR transponder signals and designed to reduce the incidence of mid-air collisions between aircraft. TCAS systems monitor the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warn pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision. An overwhelming majority of commercial and other aircraft include such TCAS systems and actively use such systems to avoid mid-air collisions by generating interrogating signals and monitoring the airspace for responses to the interrogating signals. Interrogating aircraft with older TCAS systems utilize Mode C communications and newer systems utilize Mode S communications but each is useful in identifying other aircraft in the vicinity of an interrogating aircraft. TCAS systems typically include detector systems capable of detecting interrogating signals and the signals sent responsive to the interrogating signals. In this manner, the interrogating aircraft can be alerted of another aircraft that may pose a threat.
FIG. 1 shows a Prior Art technique for alerting a friendly aircraft that it is undesirably approaching airspace being protected by a warship. Warship 2 sends radio signals 4 to approaching neutral aircraft 10 when warship 2 notices approaching neutral aircraft 10 suspiciously approaching protected airspace 8. It should be understood that various methods and techniques are known to determine if an aircraft is a neutral (i.e. not friendly or hostile) aircraft. If the aircraft is, indeed an enemy or hostile aircraft, the warship may engage the hostile aircraft such as by firing upon it. In the illustrated prior art arrangement, warship 2 may have already determined the status of neutral aircraft 10 as a neutral aircraft and may wish to alert neutral aircraft 10 not to approach protected airspace 8. In the illustrated arrangement in which radio signals 4 are used to alert the aircrew of neutral aircraft 10, problems may arise. The problems include but are not limited to technical problems associated with sending or receiving radio signals 4, inattentiveness of the aircrew or language problems between the sender and recipient of radio signals 4. Any such problems might cause neutral aircraft 10 to be unresponsive to the alerts provided by warship 2 and to continue into protected airspace 8. With neutral aircraft 10 entering protected airspace 8 unresponsive to the alerts from warship 2, warship 2 may engage neutral aircraft 10 such as by firing upon it.
It would be desirable to prevent AAW and other engagements with neutral aircraft or aircraft whose status has not been determined, by supplementing or replacing the verbal alert provided to the aircrew, using technology that is convenient and available to aircraft.